In vitro activities of carbapenem/sulbactam combination, colistin, colistin/rifampicin combination and tigecycline against carbapenem-resistant Acinetobacter baumannii

Objectives: To determine the in vitro activities and interactions of imipenem, colistin and tigecycline with old antibacterial agents against carbapenem-resistant Acinetobacter baumannii. Methods: Forty-three carbapenem-resistant A. baumannii isolates from the intensive care unit of a university hospital were collected and their MICs of imipenem, colistin and tigecycline were determined. With eight randomly selected carbapenem-resistant isolates, an in vitro time-kill study was performed for the evaluation of antibacterial activity of colistin, tigecycline, imipenem/sulbactam and colistin/rifampicin. Results: The time-kill study of colistin demonstrated bactericidal activity against A. baumannii at concentrations of 4 × MIC and 8 × MIC, whereas tigecycline showed bacteriostatic activity at all concentrations. The combination regimens of imipenem/sulbactam and colistin/rifampicin were synergistic and bactericidal at 1 × MIC. Conclusions: Imipenem/sulbactam combination, colistin and tigecycline showed good in vitro activities against carbapenem-resistant A. baumannii isolates. Even though colistin is bactericidal against carbapenem-resistant A. baumannii, the colistin/rifampicin combination is more warranted in order to be certain.